Set the drill's direction indicator (located just above the trigger) to reverse. Hold the chuck firmly with one hand and depress the trigger with the other to loosen the chuck. Continue until the chuck's jaws are opened to their widest point. Remove the bit from the chuck.
Insert an Allen wrench between the opened jaws and into the screw in the back of the chuck. Rotate the screw clockwise to loosen it. Remove the screw.
Hold the chuck firmly and depress the trigger to force the chuck loose from the drive shaft. If you cannot hold the chuck, grip it with a large pair of vise-grip pliers.
Rotate the chuck counterclockwise by hand until it comes off of the threaded drive shaft.